Verapamil reduces left ventricular (LV) outflow tract gradients in many patients with hypertrophic cardiomyopathy. We studied the mechanism of this effect using a nonimaging ECG-gated scintillation-probe before and after intravenous verapamil. Despite reduction in blood pressure, verapamil increased LV end-diastolic volume, end-systolic-volume and stroke volume; increased peak LV filllng rate; and decreased LV ejection fraction. This was associated with reduction in outflow gradient. Thus, verapamil reduces LV outflow gradient in pts with HCM presumably on the basis of increased LV volumes, which may relate to improved rapid diastolic filling.